GLP-1 (glucagon like peptide-1) is a peptide hormone secreted by human intestinal cells, which forms from proteolytic cleavage of proglucagon by an L-cell-produced protease, and is therefore named glucagon like peptide-1. Multiple studies have shown that exogenous administration of GLP-1 enhances the effects of insulin secretion. For example, when blood glucose is over 6 mmol/L, a very low concentration of GLP-1 can play a significant role in increasing insulin secretion. Once the blood glucose is restored to a normal level, the further addition of GLP-1 shall no longer have any effect on insulin secretion.
GLP-1 is present as two forms in the human body, one is GLP-1 (7-36)-NH2 (SEQ ID NO:2) which comprises of 30 amino acid residues with its C-terminal amidated. The other is GLP-1 (7-37) (SEQ ID NO:3) which comprises of 31 amino acid residues. Both GLP-1 (7-36)-NH2 and GLP-1 (7-37) may have strong enhancing effects on insulin secretion.
Previous studies have shown that GLP-1 has more advantages than insulin in the treatment of type II diabetes mellitus. GLP-1 may: 1) increase the regulation of the transcription and translation of proinsulin gene, 2) enhance the secretion of insulin and C-peptide, 3) enhance the sensitivity of cellular insulin receptor, and 4) increase the total amount of β-cells. Moreover, GLP-1 may also lower or decrease: 1) the resistance to insulin and 2) the quantity of glycohemoglobin (HbA1c), fructosamine, glucagon and fatty acids. (Nielsen J. H., et al., Regulation of beta-cell mass by hormones and growth factors, Diabetes, 50, suppl., 1:S25-9, 2001; Hui H., et al., Glucagon-like peptide 1 induces differentiation of islet duodenal homeobox-1-positive pancreatic ductal cells into insulin-secreting cells, Diabetes, 50(4):785-96, 2001).
More notably, GLP-1 is observed to be capable of enhancing β-cell division and therefore increasing the total amount of β-cells, which has not been found in any other medicines used for diabetes treatment up until now. In addition, GLP-1 is effective in those patients who have failed to respond to treatment with sulfonylurea. Furthermore, administration of GLP-1 doesn't enhance insulin secretion when the concentration of blood glucose is restored to a normal level. Thus it doesn't result in hypoglycemia. For all of the abovementioned reasons, GLP-1 is regarded as a desirable medicine to treat diabetes mellitus. This is also verified by substantial clinical studies. (Rachman J., et al., Normalization of insulin response to glucose by overnight infusion of glucagon-like peptide 1(7-36) amide in patients with NIDDM, Diabetes, 45(11):1524-30, 1996; Doyle M. E., et al., Glucagon-like peptide-1, Recent Progress in Hormone Research, 56:377-99,2001; Daniel J. Drucker, Minireview: The Glucagon-Like Peptides, Endocrinology, 142(2):521-527, 2001).
However, the cost of the chemical synthesis of GLP-1 is quite high. The retail price for reagent-grade GLP-1 is $400/mg, which greatly restricts its application in clinics. Some researchers have attempted to use genetic engineering methods to produce the recombinant GLP-1 either as a fusion protein or as a secreted protein. Yet the yield and the cost of this approach to produce GLP-1 have been far from satisfactory, thereby making large-scale production of GLP-1 at a low cost impossible at this stage.
This invention aims to develop a novel method of producing GLP-1 (7-36) and/or GLP-1 analogs by ligating genes in a tandem way. The method of the present invention can be used to simplify the process, to lower the production cost, and thereby making it possible to produce GLP-1 (7-36) and/or GLP-1 analogs on a large scale.